The device relates to a workpiece holding device for a vacuum unit, as used for fixing plate-like workpieces, such as wafers or CDs, which are to be coated in the unit, for example by-vapour deposition, and a process for its operation.
A workpiece holding device of the generic type is known (TOPAZ-RFD from Balzers Aktiengesellschaft) comprising a plurality of heat transfer plates on which workpieces intended to be coated are placed in such a way that, with the respective heat transfer plate, they enclose a gap into which a fluid heat transfer medium, in particular a gas having good thermal conductivity, is passed so that, during the coating process, the workpieces are cooled by cooling the heat transfer plates, in each case via the gas phase. The transfer plates are integrated in the baseplate, and the holder which serves for fixing the workpieces is screwed thereto.
This solution in which the adjustment of the individual layers of the workpieces, in particular of the cooling gap formed in each case between the heat transfer plate and the lower surface of the workpiece, is effected with slight deformation of the holder has various disadvantages. The screwing must be carried out manually, which is relatively complicated and time-consuming and can give rise to operating errors. For example, incorrect torques may be chosen when tightening the screws, which may lead to incorrect adjustment of the cooling gap and consequently to nonuniform cooling of the workpieces, resulting in nonuniform coating. In the case of the adjustment of the layers of the workpieces, mutual influence cannot be ruled out. Moreover, screwing can very easily result in particles, which settle on the workpieces and impair the product quality.
Accordingly, it is the object of the invention to provide a workpiece holding device of the generic type which very substantially operates automatically and with high operational safety, and a process for its operation which ensures locking and release of the holder and in which the formation of troublesome particles is virtually completely avoided. These objects are achieved by the features in the characterizing clause of claims 1 and 10, respectively. Further advantages are the very exact automatic adjustment of the gap, which takes place in each case completely independently with a plurality of workpieces, and the avoidance of the formation of troublesome particles, which leads to an improvement in the coating quality and a reduction of spoilage particularly during operation of the workpiece holding device according to the invention by the process according to the invention, there is virtually no danger of formation of troublesome particles since locking and release occur in a load-free manner and therefore virtually without friction.